Since I started using Natty I experience a strange situation at times. When I leave my system idle for 30+ mins., the screensaver Ant Spotlight remains active for specified time, then the monitor turns off after a while. Later when I am back to work on moving mouse/pressing keys on keyboard monitor powers on but I am greeted with a stuck up Ant in the screen. I won't be able to access open applications in any way. No key/mouse click works. I tried going to terminal with Ctrl+Alt+F1 and killing gnome-screensav but the Ant doesn't budge. Only way out I take is to log out with Ctrl+Alt+Backspace which works causing to lose my unsaved work.

This seems to be this or this bug. I just tried enabling "Lock screen when screensave is active" in gnome-screensaver-preferences, and it worked at least once now, i.e. I had to type my password but the screen was not blank with only the mouse cursor.
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elmichaMay 25 '11 at 20:47

The trick that helped for me was to go to the text-only terminal and then immediately back to the GUI, that is: first press CTRL+ALT+F1 and then CTRL+ALT+F7. This does not do anything about the cause of the problem but allows you to start using the GUI again.